This invention relates to the field of medical devices. In particular, the present invention relates to transcutaneous energy transfer (TET) devices.
A TET device is a device for providing electrical power to an implanted mechanical or electrical medical device, such as a bone growth stimulator, muscle stimulator, prosthetic heart or a ventricular assist device, without having to breach the skin to lead conducting wires therethrough.
In U.S. Pat. No. 3,942,535 in the name of Schulman, a transcutaneous energy transfer device for recharging a battery is disclosed. The device incorporates a telemetry link between two portions disposed on either side of a layer of tissue. An induction coil coupling allows power transfer across the tissue layer.
In U.S. Pat. No. 5,350,413 John Miller discloses a TET device with a high-energy transfer efficiency. Such a device allows for efficient transfer of energy between two coils having fixed spacing. Unfortunately, as one coil is located within a body and another coil is located outside the body, maintaining coil separation at a constant distance is difficult. Changes in coil spacing result in variation of the induced voltage and, as the distance increases, the power transfer efficiency drops off rapidly.
In an article entitled xe2x80x9cDevelopment of an Autotuned Transcutaneous Energy Transfer System, xe2x80x9d John Miller, G. Belanger, and T. Mussivand suggest an autotuning circuit to overcome this problem. The autotuning circuit compares various voltages and currents present within a driving circuit external to the body to determine a tuning requirement. Such tuning enables the tuning of energy transfer where the coil spacing varies.
It has been found that the autotuning function disclosed addresses the problem of power coupling efficiency, but fails to address a further problem of internal voltage control. In driving implanted medical devices, energy coupling efficiency and voltage control are separate but related issues to address. Better coupling efficiency results in lower operating cost and improved battery life. Voltage control results in improved device operation and increased safety. In fact, some devices will fail from excessive applied voltage.
Further, it has been found that efficiency is affected by several factors some of which include power coupling related factors such as spacing and load related factors such as medical device load requirements or faults. Unfortunately, autotuning does not address the issue of providing additional energy when required by a medical device.
It has also been found, that prolonged exposure to electromagnetic fields results in damage to human skin. Resulting damage is not believed to be linearly related to the electromagnetic field strength and exposure time. It is believed that high-energy electromagnetic fields above a certain threshold damage human skin and adjacent tissue significantly more rapidly than low energy electromagnetic fields. Since a TET device provides energy to an implanted system and some of these implanted systems require significant power, the damage to tissue such as human skin is a significant drawback to extended use of TET devices. Reducing the electromagnetic field strength and/or reducing exposure time increases tissue longevity.
Two common approaches are known for addressing the problem of tissue damage. The first, skin grafting, is a surgical technique wherein dead tissue is replaced with healthy tissue from another area of a patient""s body. Surgical techniques of this type are generally, not desirable. The second technique involves the design and implantation of lower power devices. Unfortunately, a device such as a heart pump requires significant power even when efficiently implemented.
It would be advantageous to provide a TET system that was less prone that the prior art to the problems of tissue damage.
In U.S. Pat. No. 5,350,413, John Miller further discloses an IR telemetry module for providing bi-directional communications. It is known that infra red telemetry is affected by skin pigmentation. As a transceiver disclosed by John Miller is implanted beneath a layer of skin, such considerations are important. It has been found that highly pigmented skin attenuates IR signals and renders a system as disclosed by John Miller substantially unworkable. Further, dirt and other obstructions like clothing or casings affect IR telemetry and can render it inoperable. For a television remote control, this is an acceptable limitation; for medical devices required by an individual, an inoperable TET is unacceptable.
Limitations are inherent in an IR telemetry link. IR is an optical communications means requiring an optical path between transmitter and receiver. Absent fibre or waveguides, IR telemetry is highly directional and limits a system to a single transmitter operating at a time in a direction. The directional nature of IR telemetry requires substantial alignment for optical communication.
Until recently, IR telemetry has been limited to low frequency communications. At low frequencies, it is difficult to multiplex channels, as a serial link requires higher frequencies than a true multi-channel implementation. Unfortunately, as noted above, IR telemetry is not suited to true multi-channel communications. The advent of high speed IR circuits may allow for channel multiplexing using a known technique such as time division multiplexing (TDM); however, this does not overcome previously mentioned shortcomings of IR.
Thus in an attempt to overcome these and other limitations of the prior art it is an object of the present invention to provide a TET having multiple coils for implanting at multiple locations within a patient. Each coil receives a portion of transmitted energy and thereby results in exposure of tissue at each location to electromagnetic fields of lower intensity than result from use of a single pair of coils.
In a first broad aspect, the invention seeks to provide for a transcutaneous energy transfer device for coupling with a plurality of second coils. The device includes a plurality of first coils, each first coil for performing at least one of transmitting power to and receiving power from a coil from the plurality of second coils; and, a circuit, coupled to each coil from the plurality of coils for performing one of providing power to each coil of the plurality of first coils, the power provided for transmission therefrom, and receiving and combining power from each coil of the plurality of first coils. In an embodiment, the device also includes a plurality of second coils, each coil for transmitting power to a first coil; and a second circuit, coupled to each of the second coils for providing power to each of the second coils, the power provided for transmission from the second coils.
In accordance with the invention there is provided a transcutaneous energy transfer device for coupling with a second coil. The device includes a plurality of first coils, each first coil for performing at least one of transmitting power to and receiving power from the second coil; and a circuit, coupled to each coil from the plurality of coils for performing one of providing power to each coil of the plurality of first coils, the power provided for transmission therefrom, and receiving and combining power from each coil of the plurality of first coils. In an embodiment the plurality of first coils are for receiving power; wherein the circuit is for receiving power from each coil of the plurality of first coils and for combining power from each coil of the plurality of first coils; and, wherein the plurality of first coils are for implanting within a person. In another embodiment the device also includes a plurality of first coils, each first coil for implantation beneath the skin of a patient, the coils for receiving energy in the form of electromagnetic energy transmitted from outside the patient; a plurality of second coils for transmitting power received by the second coils in the form of electromagnetic radiation; a driver circuit for providing power to the second coils; and a circuit for combining the received energy received by the first coils and for providing power to an implanted device.
In accordance with another aspect of the invention, there is provided a method of providing power from an external circuit having a plurality of primary coils to an implanted circuit having a plurality of implanted secondary coils. The method comprises the steps of: determining an amount of power to provide to the implanted circuit; dividing the amount of power into a number of portions; supplying sufficient power to each of a number of the primary coils to result in reception of a portion of the power at each of a number of the implanted secondary coils, the portions received at each of the implanted secondary coils forming the determined amount of power when combined.
In accordance with another embodiment of the invention, there is provided a transcutaneous energy transfer device. The device comprises a primary circuit comprising a plurality of primary coils coupled to at least a primary coil driver, a primary RF transceiver coupled to a plurality of the primary coils for transmitting and receiving RF signals, and primary signal filtering and extraction means for extracting information from the RF signal received by the primary RF transceiver; and a secondary circuit comprising a plurality of secondary coils, a secondary RF transceiver coupled to a plurality of the secondary coils, and secondary signal filtering and extraction means for extracting information from the RF signal received by the secondary RF transceiver.